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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN"><html><head><meta name="generator" content="HTML Tidy for Linux/x86 (vers 1 September 2005), see www.w3.org"><meta http-equiv="content-type" content="text/html; charset=us-ascii"><meta name="Generator" content="Microsoft Word 97"><title>db.* User's Guide Glossary</title></head><body><h1><a name="Glossary"></a>Glossary of <font size="2"><br></font>Database Terms</h1><p><font size="2"><strong>B-tree<br></strong> Also called a<i>multiway tree</i>, a <i>B-tree</i> is a fast data-indexingmethod that organizes the index into a multi-level set of nodes.Each node contains a sorted array of key values (the indexed data).Two important properties of a <i>B-tree</i> are that all nodes areat least half-full and that the tree is always balanced (that is,an identical number of nodes must be read in order to locate allkeys at any given level in the tree). A well-organized<i>B-tree</i> will have only three or four levels.</font></p><p><font size="2"><strong>Buffer</strong><br>An in-memory store of data read from a disk file, in which databaseoperations are performed.</font></p><p><font size="2"><strong>Cache</strong><br>A set of <i>buffers</i> used to optimize database input and outputoperations. All <b><i>db*</i></b> database input and output isperformed using a cache.</font></p><p><font size="2"><strong>Commit</strong><br>The point at which database changes made during a single<i>transaction</i> are actually written to the databasefiles.</font></p><p><font size="2"><strong>Compound key<br></strong> A <i>key</i>field composed of any combination of fields (not necessarilycontiguous) from a record. Each field of a <i>compound key</i> maybe stored in ascending or descending order.</font></p><p><font size="2"><strong>Connect</strong><br>The process of inserting a member record occurrence into a setoccurrence.</font></p><p><font size="2"><strong>Currency tables<br></strong> A table ofdatabase addresses maintained by the <b><i>db.*</i></b> runtimesystem for controlling record access and set navigation. The<i>currency tables</i> consist of the <i>current member table</i>,<i>current owner table</i>, and the <i>currentrecord</i>.</font></p><p><font size="2"><strong>Current database<br></strong> Thedatabase that is currently accessible by the <b><i>db.*</i></b>runtime functions when multiple databases have been opened. Thecurrent database is changed by the database number functionargument or by function <b>d_setdb</b>.</font></p><p><font size="2"><strong>Current member<br></strong> Contains, foreach set, the <i>database address</i> of a record occurrence thatis a valid member of that set. Usually, the <i>current member</i>of a set is the last record accessed using a set navigationfunction (<b>d_findfm</b>, <b>d_findlm</b>, <b>d_findnm</b>, or<b>d_findpm</b>).</font></p><p><font size="2"><strong>Current owner<br></strong> Contains foreach set, the <i>database address</i> of a record occurrence thatis a valid owner of that set. Usually, the <i>current owner</i> ofa set is established using the set navigation function<b>d_findco</b> or by using a currency manipulationfunction.</font></p><p><font size="2"><strong>Current record<br></strong> Contains the<i>database address</i> of the most recently accessed recordoccurrence.</font></p><p><font size="2"><strong>Data Field<br></strong> See<b>Field</b>.</font></p><p><font size="2"><strong>Data file<br></strong> A<b><i>db.*</i></b> file defined in a DDL specification thatcontains occurrences of one or more <i>record types</i>.</font></p><p><font size="2"><strong>Database</strong><br>An organized collection of related <i>files</i>.</font></p><p><font size="2"><strong>Database address<br></strong> Thelocation in the database of a record occurrence.</font></p><p><font size="2"><strong>Database Definition Language<br></strong>A programming-like language used to define the structure andcontent of a database. <b><i>db*</i></b>'s <i>Database DefinitionLanguage</i> has been designed to be used with the C programminglanguage.</font></p><p><font size="2"><strong>DDL</strong><br>See <b>Database Definition Language</b>.</font></p><p><font size="2"><strong>Deadlock</strong><br>A situation in which multiple processes accessing the same databaseeach hold locks needed by the other processes in such a way thatnone of the processes can proceed. Sometimes called <i>deadlyembrace</i>.</font></p><p><font size="2"><strong>Delete chain<br></strong> A linked listcontaining deleted records or nodes to be reused when a new recordor node is created.</font></p><p><font size="2"><strong>Dictionary</strong><br>A repository containing a definition of the content and structureof a database. It is used by the <b><i>db.*</i></b> runtime libraryfunctions for accessing and manipulating information from thatdatabase.</font></p><p><font size="2"><strong>Disconnect</strong><br>The process of removing a member record from a setoccurrence.</font></p><p><font size="2"><strong>Environment variable<br></strong> Aprogrammer-specified operating system parameter that is used toidentify configuration information to the runtimesystem.</font></p><p><font size="2"><strong>Field</strong><br>Also called <i>data field</i>, it represents the basic unit ofinformation storage in a database and is always defined to be anelement of a record. A <i>field</i> has associated with itattributes such as name, type (for example, <b>char</b> or<b>int</b>), and length. Other terms used for <i>field</i> include:<i>attribute</i>, <i>entity</i>, or <i>column</i>.</font></p><p><font size="2"><strong>File</strong><br>The primary physical storage unit into which a database isorganized. In <b><i>db.*</i></b>, files are used to store recordsand keys.</font></p><p><font size="2"><strong>Hierarchical database model<br></strong>A data representation in which the relationships between recordtypes are formed from parent-child structures, such that a recordtype may have many child relationships but only one parentrelationship.</font></p><p><font size="2"><strong>Index</strong><br>A set of key values through which rapid retrieval of a record isprovided, similar to the index of a book. The term is often usedsynonymously with <i>key file</i>.</font></p><p><font size="2"><strong>Key file<br></strong> A file that onlycontains keys. It may, in fact, contain more than one <i>index</i>because multiple key types can be contained in a single<b><i>db.*</i></b> <i>key file</i>.</font></p><p><font size="2"><strong>Key scan<br></strong> The process ofperforming an ordered traversal through all (or a subset of all)occurrences of a given key field.</font></p><p><font size="2"><strong>Key</strong><br>A <i>field</i> through which rapid and/or sorted access to a recordis desired.</font></p><p><font size="2"><strong>Leaf node<br></strong> A <i>node</i> atthe lowest level of a <i>B-tree</i>, having no childnodes.</font></p><p><font size="2"><strong>Lock</strong><br>A multi-user database <i>synchronization</i> mechanism, used toprevent simultaneous updates to shared data. Locks can be appliedto the entire database, to files, or to recordoccurrences.</font></p><p><font size="2"><strong>Logging</strong><br>The process of making a copy of the database changes made during a<i>transaction</i> prior to a <i>commit</i>. <i>Logging</i> is usedto support the ability to perform a <i>recovery</i> in the event afailure occurs during a <i>commit</i>.</font></p><p><font size="2"><strong>Many-to-many relationship<br></strong> Arelationship between two record types, A and B, such that for each⌨️ 快捷键说明
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